Tuesday, January 15, 2013

How big is the GIANT Squid Giant Axon?

With all the hubbub about the first ever video of an attacking giant squid in the wild about to unveiled, I started wondering about the giant axon of the giant squid... I mean it would be huge right?...

Here is a really nice 5 minute video showing how with (by today's standards) very crude techniques, the electrical signal could be recorded from these axons.

So the squid giant axon is neat, and modern neurophysiology would probably not exist with out it. But what about the GIANT squid giant axon? Wouldn't that be an electrophysiologist's dream?

If it scaled proportionally to say, mantle length, the 1foot long Atlantic squid with a 1mm diameter axon would become a 16 foot long GIANT squid with a 16mm giant axon.
Let's think about this for a minute, 16mm is about 5/8 of an inch.

That is like the diameter of a dime! For those not familiar with US coins, it's like the size of a bead on a necklace... a big bead, like a nice-sized pearl. Basically HUGE considering that most axons in vertebrates are not even visible without a microscope.

However,before you all start running out to hunt the giant squid for its precious precious axon...the truth is that the giant squid does not have a super-giant dime-sized axon. The giant squid axon actually has a smaller diameter than the 'normal' squid axon. Surprising right?
Do the giant squid just have more axons there, so they don't need one gigantic one? Or is this axon somehow magically myelinated (probably not)? Or does the giant squid just not need one?

First, let me explain that this information was pretty hard to come by and basically anecdotal. I watched a few dissections of giant squid. And while these were really amazing (look at the hooks on the colossal squid's tentacles!), they said very little about the giant axon or how it was modified in these larger animals.

This information comes from a comment quoting JZ Young at a 1977 symposium describing his dissection of a 125cm (about 4 feet) long giant squid. I could not get access to this manuscript, so I have to trust the commenter with his quote:

“Everyone wants to know whether giant squids have giant giant fibres. We
have no material of the central nervous system but some years ago I was
able to dissect the stellate ganglion of an animal washed up at
Scarborough in 1933 and sent to the British Museum. The mantle length
was 125 cm. The nerves of the mantle muscles are arranged in this genus
differently from any other I have seen. Those in the front part of the
mantle arise from a relatively small stellate ganglion, in the usual
way. The hinder part of the mantle, perhaps more than half of the whole,
is suspended from a distinct median nerve, running with the fin nerve
and giving off a series of branches to the mantle.
Each of the nerves arising from the ganglion contains one or two large
fibres, ranging in diameter from about 80 micrometers in the more
anterior ones to a maximum of 250 micrometers further back. The median
nerve was further preserved but one fibre of about 250 micrometers could
be seen. Two of the more posterior branches contained fibres of about
200 micrometers each. None of the nerves examined contained the
exceptionally large fibres reported by Aldrich & Brown (1967). We
may conclude that Architeuthis is not an especially fast-moving animal.
This would agree with evidence that it is neutrally buoyant with a high
concentration of ammonium ions in the mantle and arms (Denton, 1974).”

Young explains that the axon network is set up differently in the giant squid (Architeuthis). He reasons that because the axon is not especially large, it could only conduct so fast, and therefore the fast escape reflex which it causes in the normal squid is just not that fast in the giant squid. This sort of makes sense, in that the giant squid might not benefit from escape as much as the normal squid. The giant squid might be better served by having razor sharp teeth on its suckers or terrifying pain causing-hooks so it could fight away a predator.

The biggest axon award goes to the Humboldt Squid which has an axon the 'size of spaghetti.'